This invention relates in general to lubricant supply systems for thrust bearings of the type used in large rotating shaft machines; and, in particular, to means responsive to axial thrust within the machine which apportion the lubricant supply to either an active or inactive bearing.
Rotating shaft machines such as turbines include stationary thrust bearings for preventing excess axial movement of the rotating shaft. The rotating shaft has a thrust collar which is rotatable with the shaft and which is positioned axially between an "active" and an "inactive" thrust bearing. Lubricating fluid is sent to both bearings to provide a lubrication film on the face of each thrust bearing which then supports the thrust collar as the shaft moves in either axial direction. The "active" thrust bearing is the bearing expected to ordinarily carry the higher loads most of the time whereas the "inactive" bearing would carry loads part of the time.
The problem which the present invention solves is to minimize the total lubricant flow rate to the two thrust bearings while still providing for a variety of operating conditions which may cause the normally inactive thrust bearing to become loaded.
In the prior art, it is a common practice in turbomachinery thrust bearing design to establish the size of the required normally active thrust bearing based upon some assumed steady-state operating condition of the machine. Axial thrust forces acting upon various components of a rotor are estimated, including, for example; the blades, wheel disks etc. and a net axial thrust is determined for this steady-state condition. The net axial thrust is typically the small difference between two large values thrusting the rotor in the fore and aft directions. It is well known to designers that this net axial thrust calculation may be subject to considerable error. Nevertheless, the normally active thrust bearing is generally sized based upon such a calculation of net thrust load and the allowable load per square inch of thrust bearing area. The lubricant flow rate to this normally active bearing is then established from considerations of its size, load and rubbing velocity.
In the design of heavy duty gas turbines, it is a usual practice to provide a normally inactive bearing smaller than the normally active bearing based upon the presumption that the inactive thrust will not exceed 50 percent of the design point active thrust load. The inactive thrust bearing size and the required flow rates are therefore based upon this premise. A total flow to the thrust bearings is supplied based upon the needs of the full-size active thrust bearing and the reduced capacity of the inactive thrust bearing. The usual thrust bearing lubricant supply arrangement includes a common header which supplies a journal bearing and two thrust bearings. The flow rate to each of the thrust bearings is controlled by different size orifices or drilled holes in the castings through which the lubricant must pass from the inlet header.
The design assumption for sizing the inactive thrust bearing does result in the benefits of reduced lubricant flow. Under certain conditions, however, the lubricant supplied to the inactive bearing may be inadequate or marginal for the imposed thrust load. As previously noted, the calculation of net thrust load is inherently subject to error even at the machine design point condition. Under off-design machine operation, or under transient machine operation, or with the wear of some critical components, it is possible for the normally inactive bearing to become subject to a high thrust force. In such a case, it is desirable to have more flow to the inactive bearing than is normally provided.
It is a universal practice to have both active and inactive thrust bearings on a turbomachine rotor. In cases where turbomachines have equally-sized active and inactive, it is common practice to supply lubricant at a rate sufficient for both bearings.